In prior art transmission devices rolling contact between planets and raceways inevitably requires circular arcs for the rolling contact surfaces, that is the generatrix of the curved surfaces in rolling contact with one another. This is especially true of rolling contact transmission devices of the toroidal cavity type but also of these of the half-toroidal cavity type. This means that the traction coefficient, that is the ratio between the traction force exerted between plane and raceway and the contact force between these members can only be controlled by using separate force generators (each as hydraulic pressure) and designs allowing maximum efficiency at different ratios are not available. This has the disadvantage that the contact force between planets and raceways is not optimum at all ratios.
In a continuously variable rolling contact transmission device of the type defined above the input to the device may be applied via the radially inner races and the output from the device taken from the planets via planet followers or a planet carrier, with the outer race constituting the stationary component. The high gear ratio is then achieved with the two components of the radially outer race located at their position of maximum spacing whilst the parts of the inner race are located as close to one another as possible so that the planets are, effectively, “squeezed” to their greatest radial position. Such a transmission may be referred to as a quasi-toroidal rolling contact transmission. It will be appreciated that in such a transmission the roles of input and output shaft can be reversed and, in the design in question, the roles of the three components, namely radially inner races, planets, which in different embodiments includes planet followers and planet carriers, and radially outer races are all interchangeable so that any one of them may be held stationary and the other two used either as the input or the output member. A configuration with the outer race stationary will be described in more detail below but it is to be understood that the invention is not intended to be limited to such a configuration and is equally applicable to others.